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DEPARTMENT OF COMMERCE 



Scientific Papers 



OF THE 



Bureau of Standards 

S. W. STRATTON. Director 



No. 343 



LOCATION OF FLAWS IN RIFLE-BARREL STEEL 
BY MAGNETIC ANALYSIS 

BY 

R. L. SANFORD, Associate Physicist 

and 

WM. B. KOUWENHOVEN, Consulting Engineer 
Bureau of Standards 



ISSUED OCTOBER 3, 1919 



jCj.Xlb"'^ ^ 




PRICE, 5 CENTS 

Sold only by the Superintendent of Documents, Government Printing Office 
Washington, D. C. 



WASHINGTON 

GOVERNMENT PRINTING OFFICE 

1919 



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53 



D« Of D. 
ocr 26 1919 



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LOCATION OF FLAWS IN RIFLE-BARREL STEEL BY 

MAGNETIC ANALYSIS 



By R. L. Sanford and Wm. B. Kouwenhoven 



CONTENTS 

Page 

I. Introduction 219 

II. Theory 219 

III. Description of apparatus and procedure 221 

IV. Preliminary study and adjustment 223 

V. Experimental results , ■ 225 

VI. Summary ■ 228 

I. INTRODUCTION 

One of the practical applications of magnetic analysis consists 
of the detection of flaws in bar stock used in the manufacture of 
steel products. At the request of the Ordnance Department of 
the Army and the Winchester Repeating Arms Co., an investiga- 
tion was undertaken dtnring the war with the end in view of apply- 
ing this method of magnetic analysis to the testing of rifle-barrel 
steel. 

In view of the fact that flaws, generally consisting of pipes or 
slag inclusions, interfere with the drilling of the barrels or may 
possibly affect their strength, it was considered that a nonde- 
structive test which would detect and locate such flaws before 
further work had been done on the barrels would prove to be of 
great value. Such a method of inspection would make possible 
not only the rejection of faulty material, but also the acceptance 
of all the satisfactory bars in a given shipment and thus effect a 
great saving both of material and labor. It is the object of this 
paper to describe the apparatus used in the investigation and to 
present the results thus far obtained. 

II. THEORY 

The method employed was that of the determination of the 
degree of magnetic uniformity along the length of the bars, based 
upon the theory that if a bar is uniform magnetically along its 

120130°— 19 219 



220 



Scientific Papers of the Bureau of Standards 



[Vol. IS 



length, it is also uniform mechanically. A number of barrel 
forgings were first tested by a point-by-point method originally 
used for the examination of bars intended for magnetic standards 
and which has already been described.^ Fig. i shows a specimen 
curve obtained by this method, and Fig. 2 is a photograph showing 




-S -^ O 4 8 12 

Di5Tamc£ along Bar-cm 



Fig. I. — Magnetic uniformity curve. Piped sample 

the flaw which was indicated by this ctirve. Since this method 
is not adapted to the examination of very long bars, and is too time- 
consuming for a commercial test, it was decided to use a somewhat 
different method, substantially similar to that used by Burrows ^ 
for the examination of steel rails. In this method the magnetizing 

■ Sanford, The Determination of the Degree of Uniformity of Bars for Magnetic Standards, Bureau of 
Standards Scientific Papers No. 295. 

* Burrows, Correlation of the Magnetic and Mechanical Properties of Steel, Bureau of Standards Scien- 
tific Papers No. 272, p. 203. 



Scientific Papers of the Bureau of Standards, Vol. 15 




Fig. 2. — Photograph of the flaw indicated in the curve of Fig. I 



Scientific Papers of the Bureau of Standards, Vol. 15 




Fig. 3. — Apparaiuj a^^cniblcd at the Bureau of Standards 



^^wmkrven] Magnetic Analysis of Rifle-Barrel Steel 221 

force is applied by means of a solenoid which surrounds the bar 
and travels along its length. Mounted within this magnetizing 
solenoid is a test coil by means of which variations in magnetic 
flux within the bar can be measured. If the bar is magnetically 
uniform along its length its permeabiHty is constant for a given 
magnetizing force and the magnetic flux at each point as the 
solenoid is moving along is constant. If this is the case, there 
will be no electromotive force induced in the test coil as the solenoid 
travels the length of the bar. If, on the other hand, the permea- 
bility is not constant, the flux will vary and a corresponding 
electromotive force will be induced in the test coil which, if the 
coils are moved at a constant speed along the bar, is proportional 
to the change in flux. If, instead of using a single test coil in the 
manner just described, we use two test coils connected in series 
opposition we obtain a result that is practically not affected by 
sUght variations in the magnetizing current during a run, as any 
variations in flux linked with one coil is neutralized by correspond- 
ing changes in the other. 

III. DESCRIPTION OF APPARATUS AND PROCEDURE 

Fig. 3 is a photograph of the apparatus as set up at the Bureau 
of Standards for preliminary experiments before it was taken to 
the Winchester plant at New Haven for test under factory con- 
ditions. The bar to be examined is clamped at the centers of two 
triangular end plates of cast iron. These end plates are sup- 
ported by three wrought-iron pipes which also constitute the 
return circuit for the magnetic flux induced in the test bar. The 
magnetizing solenoid, which is shown in more detail in Fig. 4, 
is supported between the pipes by means of cords running over 
pulleys and carrying counterweights which hang inside the sup- 
porting pipes. One of these cords is continuous and mns over a 
drum mounted on the shaft of a small electric motor. By means 
of this arrangement the coils can be run up and down along the 
length of the bar. Magnetizing current is suppUed to the solenoid 
by means of a storage battery and regulated by means of sliding 
rheostats. The guiding rollers shown in Fig. 4 were later arranged 
to bear on the test bar instead of on the iron pipes, as it was 
found that many of the bars we're not straight. The test coils 
are mounted on a separate tube and their position is adjustable. 
These test coils have 500 turns each, and are connected through 
suitable resistances to the galvanometer shown at the right of the 



222 



Scientific Papers of the Bureau of Standards [Voi. 15 



apparatus in the photograph. Deflections of the galvanometer 
are observed by means of a spot of Hght reflected from its mirror 
onto a ground-glass scale. Permanent records of these deflec- 
tions are made by means of a photographic arrangement which 
consists of a long light-tight box upon one end of which is moimted 
an ordinary oscillograph drum which carries the photographic 
film. This drum is rotated at the proper speed by means of a 
belt connected to the driving motor of the apparatus. By means 
of contacts located at i-foot intervals on the driving cord, a light 
is flashed inside the box which makes a record on the film for each 
foot of travel, and thus affords a means for locating the position 



ma6metizim6 
Circuit 



Test Circuit 




Fig. 5. — Diagram of electrical connections 

on the bar of any observed nonuniformity. A diagram of the 
electrical connections is given in Fig. 5. 

When a photographic record indicating the magnetic uniformity 
of a bar is to be made, the procedure is as follows: The bar is 
clamped in the apparatus, the galvanometer circuit is then closed, 
and the drum carrying the photographic film is given one com- 
plete revolution. The spot of light reflected from the galvanome- 
ter mirror thereby traces a straight line which serves as the refer- 
ence axis. The switch M is then closed and the magnetizing 
current is adjusted to the proper value by means of the regulating 
rheostat R. With the galvanometer connected either to the single 
test coil or the differential coils as desired, the driving motor is 



Scientific Papers of the Bureau of Standards, Vol. 15 




Fig. 4. — Magnetizing solenoid and test coils 



Sanford T 

Kouwenhaveni 



Magnetic Analysis of Rifle-Barrel Steel 



223 

then started and the coil is run the length of the specimen with 
the fihn holder rotating at a uniform speed. Most of the records 
have been made by running the coils in one direction with the 
galvanometer connected to the single test coil, and in the other 




3 -(V 5 

Fig. 6. — Record with single test coil 



direction with the galvanometer connected to the differential coils. 
Fig. 6 shows a record taken by the use of the single coil, and 
Fig. 7 shows the corresponding record taken with the differential 
coils. A rectangle is drawn on each of these records to show the 
position and extent of a strip of transformer iron which was 
attached to the bar in order to give the effect of a flaw. 




Fig. 7. — Record with differential test coil 

IV. PRELIMINARY STUDY AND ADJUSTMENT 

After the apparatus was completed and set up in the laboratory, 
it was necessary to consider a number of points in connection with 
its operation and to decide upon the proper adjustment of the test 
coils. The points to be considered included the proper flux 
density, B, in the specimen, the proper speed for the moving coils, 
the period of the galvanometer and, as just mentioned, the best 
location of the test coils. As a result of observations taken under a 



224 Scientific Papers of the Bureau of Standards [Voi. ts 

great variety of conditions, it was found that a flux density of 
approximately 15 000 gausses gives the best results. The speed 
of travel finally adopted was approximately X foot per second. 
It is necessary in order to insure that the record gives a true indi- 
cation of the condition of the specimens that the galvanometer 
have a fairly short period. If the period is too long, the galvanome- 
ter does not follow closely the changes in the induced electromotive 
force. A period of approximately one second was found to be 
satisfactory. The photograph of the test coils shows only two coils 
in position. For convenience, however, a third coil was made, 



CeT 5h ORT - BLUNT E h 05. 



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One TAPEWN6 AND 
12" ► ONE BLUNT END. 



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Fig. 8. — Dimensions of added strips 

and the differential coils were located 10 cm. apart and equidistant 
from the single coil which was located at the middle of the mag- 
netizing solenoid. With this symmetrical arrangement, records 
could be duplicated by running the coils in either direction. 

A preliminary exploration to determine the flux distribution 
along the specimen for different positions of the magnetizing 
solenoid was made by a point-by-point method, using the single 
test coil connected to a ballistic galvanometer. Readings were 
taken upon reversal of the magnetizing cturent. The result of 
this exploration showed that, for a given magnetizing current, the 
flux is constant at different points along a uniform bar except for 
the regions very near the ends. 



fc^wfnhaven] Magnetic Analysis of Rifle-Barrel Steel 225 

In order to study the effect of flaws varying in extent and kind, 
a number of records were made on a bar previously found to be 
uniform, to which were attached strips of transformer steel of 
various shapes and sizes. This procedure was necessary, because 
of the difficulty of producing longitudinal flaws by artificial means. 
Fig. 8 shows the shapes and dimensions of the strips thus used. 
Figs. 9 to 12, inclusive, are records obtained in this way. The 
location and shape of the added strip is indicated in each case 
upon the record. The figures show records taken both by means 
of the single test coil and by means of the differential coils. Figs. 
13 and 14 show the effect of various treatments on a bar originally 
uniform. The treatments given, and the locations, are indicated 
in the figures. It was at first feared that, due to the sensitiveness 
of the method, spm-ious indications would be obtained for bars 
which had been slightly bent during shipment and handling at the 
factory. The result of this last test, however, indicates that such 
is not the case. 

After the preliminary experiments just described, the apparatus 
was shipped to the plant of the Winchester Repeating Arms Co., 
at New Haven, and there set up for final trial. 

V. EXPERIMENTAL RESULTS 

The greatest difficulty in this line of investigation lies in the 
interpretation of the results. This is due to the fact that there 
are many causes which may produce magnetic inhomogeneity 
and it is difficult to differentiate between them. The work at 
New Haven was done with the end in view of obtaining data which 
would establish the amount of variation and the type of curve 
which accompanies a pipe. The procedure was to make records 
of bars which in a preliminary test showed large variations. 
These bars were chosen from lots of steel which had previously 
been rejected as the result of tests in the drilling shop. It is an 
interesting fact that even though this lot of steel had previously 
been rejected on account of pipes, not a single pipe was discovered 
in the drilling tests on samples for which records of the magnetic 
uniformity had been obtained. This is true of all the steel 
examined up to March 31, 191 9. 

Figs. 15 and 16 show records of the degree of magnetic homo- 
geneity of four bars of steel. These records were made with 
the differential test coils and with a fairly low sensitivity of the 
galvanometer. The portions of these bars from which barrel 



226 Scientific Papers of the Bureau of Standards [Voi.15 





K^ZLhmet] Magnetic Analysis of Rifle-Barrel Steel 227 






228 



Scientific Papers of the Bureau of Standards 



{Vol. IS 



lengths were cut are indicated in the figure. Barrels A, B, C, 
and D, cut from bars i and 9 and those cut from the entire length 
of bars 2 and 4 were sent to the shop for drilling tests. Barrels 
B and D gave trouble in drilling and each destroyed the edge of 




Fig. 13 

a drill, thus necessitating the use of a new drill to finish the bore. 
None of the other barrels gave trouble and the inside surfaces of 
all were smooth and bright. In view of the fact that additional 
data are necessary in order to draw satisfactory conclusions, the 
Winchester Repeating Arms Co. is continuing the investigation. 




Fig. 14 
VI. SUMMARY 

This paper describes an investigation which was undertaken 
for the ptupose of determining whether an application of magnetic 
analysis was practicable for the detection of flaws in rifle-barrel 
steel. By means of apparatus especially constructed for thie 
purpose a large ntunber of bars were explored for magnetic uni- 
formity along their length. In spite of the fact that these bars 
were taken from material which had previously been rejected as 
the result of drilling tests, not one was found which contained a 
pipe. The results obtained, however, demonstrated that the 



IcMwLhoveiJi Magnetic Analysis of Rifle-Barrel Steel 229 









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230 Scientific Papers of the Bureau of Standards [Voi. 15 

method is amply sensitive to detect and locate flaws. Fm*ther 
study is necessary to determine to what degree the sensitivity of 
the apparatus should be reduced in order not to cause the rejection 
of material which is satisfactory for all practical purposes, and 
also to determine the type and magnitude of the effect which will 
be produced by a pipe. For this reason the work is being con- 
tinued by the Winchester Repeating Arms Co., who cooperated in 
the investigation and at whose plant the apparatus has been 
installed. 

The authors wish to take this opportunity to acknowledge 
their indebtedness to J. S. Gravely, M. F. Fischer, and J. S. 
Becker for their valuable assistance in carrying out this investi- 
gation. 

Washington, April 14, 191 9. 



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